Semiconductor device with programmable unit and method for fabricating the same

ABSTRACT

The present application discloses a semiconductor device with a programmable unit and a method for fabricating the semiconductor device. The semiconductor device includes a substrate, a bottom conductive layer positioned in the substrate, an insulation layer positioned on the substrate, a first conductive layer positioned on the insulation layer and above the bottom conductive layer, a second conductive layer positioned on the insulation layer and above the bottom conductive layer and spaced apart from the first conductive layer, a conductive plug electrically coupled to the bottom conductive layer, and a top conductive layer electrically coupled to the first conductive layer and the second conductive layer. The first conductive layer has a first work function and the second conductive layer has a second work function different from the first work function. The bottom conductive layer, the insulation layer, the first conductive layer, and the second conductive layer together configure a programmable unit.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device and a methodfor fabricating the semiconductor device, and more particularly, to asemiconductor device with a programmable unit and a method forfabricating the semiconductor device with the programmable unit.

DISCUSSION OF THE BACKGROUND

Semiconductor devices are used in a variety of electronic applications,such as personal computers, cellular telephones, digital cameras, andother electronic equipment. The dimensions of semiconductor devices arecontinuously being scaled down to meet the increasing demand ofcomputing ability. However, a variety of issues arise during thescaling-down process, and such issues are continuously increasing.Therefore, challenges remain in achieving improved quality, yield,performance, and reliability and reduced complexity.

This Discussion of the Background section is provided for backgroundinformation only. The statements in this Discussion of the Backgroundare not an admission that the subject matter disclosed in this sectionconstitutes prior art to the present disclosure, and no part of thisDiscussion of the Background section may be used as an admission thatany part of this application, including this Discussion of theBackground section, constitutes prior art to the present disclosure.

SUMMARY

One aspect of the present disclosure provides a semiconductor deviceincluding a substrate, a bottom conductive layer positioned in thesubstrate, an insulation layer positioned on the substrate, a firstconductive layer positioned on the insulation layer and above the bottomconductive layer, a second conductive layer positioned on the insulationlayer and above the bottom conductive layer and spaced apart from thefirst conductive layer, a conductive plug electrically coupled to thebottom conductive layer, and a top conductive layer electrically coupledto the first conductive layer and the second conductive layer. The firstconductive layer has a first work function and the second conductivelayer has a second work function different from the first work function.The bottom conductive layer, the insulation layer, the first conductivelayer, and the second conductive layer together configure a programmableunit.

In some embodiments, the first conductive layer and the secondconductive layer are formed of doped polycrystalline silicon, dopedpolycrystalline silicon germanium, or a combination thereof, and thefirst conductive layer and the second conductive layer have a sameelectrical type.

In some embodiments, the bottom conductive layer is formed of dopedsilicon, doped germanium, doped silicon germanium, or a combinationthereof, and the bottom conductive layer has a same electrical type asthe first conductive layer and the second conductive layer.

In some embodiments, the semiconductor device includes a well regionpositioned in the substrate and surrounding the bottom conductive layer.The well region has an electrical type opposite to that of the bottomconductive layer.

In some embodiments, the semiconductor device includes assistance layerscovering the first conductive layer and the second conductive layer. Theassistance layers are formed of titanium silicide, nickel silicide,nickel platinum silicide, tantalum silicide, or cobalt silicide.

In some embodiments, the semiconductor device includes spacerspositioned on sidewalls of the first conductive layer and. sidewalls ofthe second conductive layer.

In some embodiments, the bottom conductive layer is formed of tungsten,aluminum, titanium, copper, or a combination thereof.

In some embodiments, the first conductive layer and the secondconductive layer are formed of different materials, the first conductivelayer is formed of tungsten, cobalt, zirconium, tantalum, titanium,aluminum, ruthenium, copper, platinum, or a combination thereof, and thesecond conductive layer is formed of tungsten, cobalt, zirconium,tantalum, titanium, aluminum, ruthenium, copper, platinum, or acombination thereof.

Another aspect of the present disclosure provides a semiconductor deviceincluding a substrate, a bottom conductive layer positioned in thesubstrate, a first gate structure including a first gate dielectriclayer positioned on the bottom conductive layer, a first work functionlayer positioned on the first gate dielectric layer, and a first fillerlayer positioned on the first work function layer, a second gatestructure including a second gate dielectric layer positioned on thebottom conductive layer and spaced apart from the first gate dielectriclayer, a second work function layer positioned on the second gatedielectric layer, and a second filler layer positioned on the secondwork function layer, a conductive plug electrically coupled to thebottom conductive layer, and a top conductive layer electrically coupledto the first gate structure and the second gate structure. The firstwork function layer has a first work function. The second work functionlayer has a second work function different from the first work function.The bottom conductive layer, the insulation layer, the first gatestructure, and the second gate structure together configure aprogrammable unit.

In some embodiments, the first gate dielectric layer and the second gatedielectric layer have a same thickness.

In some embodiments, the semiconductor device includes spacerspositioned on two sides of the first gate structure and on two sides ofthe second gate structure.

In some embodiments, the semiconductor device includes a first wettinglayer positioned between the first work function layer and the firstfiller layer. The first wetting layer is formed of titanium, tantalum,nickel, or cobalt.

In some embodiments, the semiconductor device includes a first barrierlayer positioned between the first wetting layer and the first fillerlayer. The first barrier layer is formed of titanium nitride, tantalumnitride, or a combination thereof.

In some embodiments, the first gate dielectric layer and the second gatedielectric layer have U-shaped cross-sectional profiles.

In some embodiments, the bottom conductive layer is formed of dopedsilicon, doped germanium, doped silicon germanium, or a combinationthereof.

In some embodiments, the bottom conductive layer is formed of tungsten,aluminum, titanium, copper, or a combination thereof.

Another aspect of the present disclosure provides a semiconductor deviceincluding a substrate, a bottom conductive layer positioned in thesubstrate, an insulation layer positioned on the substrate, a first gatestructure positioned on the insulation layer and above the bottomconductive layer and including a first work function layer and a firstfiller layer, a second gate structure positioned on the insulation layerand above the bottom conductive layer, spaced apart from the first gatestructure, and including a second work function layer and a secondfiller layer, a conductive plug electrically coupled to the bottomconductive layer, and a top conductive layer electrically coupled to thefirst gate structure and the second gate structure. The first workfunction layer has a first work function. The second work function layerhas a second work function different from the first work function. Thebottom conductive layer, the insulation layer, the first gate structure,and the second gate structure together configure a programmable unit.

Another aspect of the present disclosure provides a method forfabricating a semiconductor device including providing a substrate,forming a bottom conductive layer in the substrate, forming aninsulation layer on the substrate, forming a first conductive layer onthe insulation layer and above the bottom conductive layer, and having afirst work function, forming a second conductive layer on the insulationlayer and above the bottom conductive layer, spaced apart from the firstconductive layer, and having a second work function different from thefirst work function. The bottom conductive layer, the insulation layer,the first conductive layer, and the second conductive layer togetherconfigure a programmable unit.

In some embodiments, the steps of forming the first conductive layer andforming the second conductive layer include forming a first intermediateconductive layer and a second intermediate conductive layer on theinsulation layer, forming a first mask layer to cover the secondintermediate conductive layer and expose the first intermediateconductive layer, performing a first implantation process to turn thefirst intermediate conductive layer into the first conductive layer,removing the first mask layer, forming a second mask layer to cover thefirst conductive layer and expose the second intermediate conductivelayer, performing a second implantation process to turn the secondintermediate conductive layer into the second conductive layer, andremoving the second mask layer. The first implantation process and thesecond implantation process are performed with different dopantconcentrations.

In some embodiments, the bottom conductive layer, the first conductivelayer, and the second conductive layer have a same electrical type.

Due to the design of the semiconductor device of the present disclosure,the resistance of the programmable unit after a programming procedurecan be fine-tuned by controlling the programming voltage applied. Inaddition, various of programming voltages can be selected and applied toprogram the programmable unit. Furthermore, the programmable unit can beprogrammed by a relatively smaller (or shallower) voltage. As a result,more surface area can be provided for other functional elements such aslogic function elements.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter, and form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present disclosure. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the disclosure as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates, in a flowchart diagram form, a method forfabricating a semiconductor device in accordance with one embodiment ofthe present disclosure;

FIGS. 2 to 8 illustrate, in schematic cross-sectional view diagrams, aflow for fabricating the semiconductor device in accordance with oneembodiment of the present disclosure;

FIGS. 9 to 13 illustrate, in schematic cross-sectional view diagrams,semiconductor devices in accordance with some embodiments of the presentdisclosure;

FIG. 14 illustrates, in a flowchart diagram form, a method forfabricating a semiconductor device in accordance with another embodimentof the present disclosure;

FIGS. 15 to 28 illustrate, in schematic cross-sectional view diagrams, aflow for fabricating the semiconductor device in accordance with anotherembodiment of the present disclosure;

FIGS. 29 and 30 illustrate, in schematic cross-sectional view diagrams,semiconductor devices 1H and 1I in accordance with some embodiments ofthe present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

It should be understood that when an element or layer is referred to asbeing “connected to” or “coupled to” another element or layer, it can bedirectly connected to or coupled to another element or layer, orintervening elements or layers may be present.

It should be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. Unless indicated otherwise, these terms areonly used to distinguish one element from another element. Thus, forexample, a first element, a first component or a first section discussedbelow could be termed a second element, a second component or a secondsection without departing from the teachings of the present disclosure.

Unless the context indicates otherwise, terms such as “same,” “equal,”“planar,” or “coplanar,” as used herein when referring to orientation,layout, location, shapes, sizes, amounts, or other measures do notnecessarily mean an exactly identical orientation, layout, location,shape, size, amount, or other measure, but are intended to encompassnearly identical orientation, layout, location, shapes, sizes, amounts,or other measures within acceptable variations that may occur, forexample, due to manufacturing processes. The term “substantially” may beused herein to reflect this meaning. For example, to items described as“substantially the same,” “substantially equal,” or “substantiallyplanar,” may be exactly the same, equal, or planar, or may be the same,equal, or planar within acceptable variations that may occur, forexample, due to manufacturing processes.

In the present disclosure, a semiconductor device generally means adevice which can function by utilizing semiconductor characteristics,and an electro-optic device, a light-emitting display device, asemiconductor circuit, and an electronic device are all included in thecategory of the semiconductor device.

It should be noted that, in the description of the present disclosure,above (or up) corresponds to the direction of the arrow of the directionZ, and below (or down) corresponds to the opposite direction of thearrow of the direction Z.

It should be noted that the terms “forming,” “formed” and “form” maymean and include any method of creating, building, patterning,implanting, or depositing an element, a dopant or a material. Examplesof forming methods may include, but are not limited to, atomic layerdeposition, chemical vapor deposition, physical vapor deposition,sputtering, co-sputtering, spin coating, diffusing, depositing, growing,implantation, photolithography, dry etching, and wet etching.

FIG. 1 illustrates, in a flowchart diagram form, a method 10 forfabricating a semiconductor device 1A in accordance with one embodimentof the present disclosure. FIGS. 2 to 8 illustrate, in schematiccross-sectional view diagrams, a flow for fabricating the semiconductordevice 1A in accordance with one embodiment of the present disclosure.

With reference to FIGS. 1 and 2, at step S11, a substrate 101 may beprovided and a bottom conductive layer 103 may be formed in thesubstrate 101.

With reference to FIG. 2, the substrate 101 may be a bulk semiconductorsubstrate, a multi-layered or gradient substrate, or the like. Thesubstrate 101 may include a semiconductor material, such as an elementalsemiconductor including silicon and germanium; a compound or alloysemiconductor including silicon carbide, silicon germanium, galliumarsenide, gallium phosphide, gallium arsenide phosphide, aluminum indiumarsenide, aluminum gallium arsenide, gallium indium arsenide, indiumarsenide, gallium indium phosphide, indium phosphide, indium antimonide,or gallium indium arsenide phosphide; or a combination thereof. Thesubstrate 101 may be doped or undoped.

With reference to FIG. 2, the bottom conductive layer 103 may be formedin the substrate 101 and the top surface of the bottom conductive layer103 may be substantially coplanar with the top surface of the substrate101. In some embodiments, the bottom conductive layer 103 may define anoperation area of a programmable unit.

In some embodiments, the bottom conductive layer 103 may be formed by animplantation process. That is, the bottom conductive layer 103 may beturned from a portion of the substrate 101. The dopants of theimplantation process may include p-type impurities (dopants) or n-typeimpurities (dopants). The p-type impurities may be added to an intrinsicsemiconductor to create deficiencies of valence electrons. In asilicon-containing substrate, examples of p-type dopants, i.e.,impurities include but are not limited to boron, aluminum, gallium, andindium. The n-type impurities may be added to an intrinsic semiconductorto contribute free electrons to the intrinsic semiconductor. In asilicon-containing substrate, examples of n-type dopants, i.e.,impurities, include but are not limited to antimony, arsenic, andphosphorous. In some embodiments, the dopant concentration of the bottomconductive layer 103 may be between about 1E19 atoms/cm{circumflex over( )}3 and about 1E21 atoms/cm{circumflex over ( )}3. After theimplantation process, the bottom conductive layer 103 may have anelectrical type such as n-type or p-type.

In some embodiments, an anneal process may be performed to activate thebottom conductive layer 103. The temperature of the anneal process maybe between about 800° C. and about 1250° C. The anneal process may havea process duration between about 1 millisecond and about 500milliseconds. The anneal process may be, for example, a rapid thermalanneal, a laser spike anneal, or a flash lamp anneal.

With reference to FIGS. 1 and 3, at step S13, an insulation layer 105may be formed on the substrate 101.

With reference to FIG. 3, the insulation layer 105 may be formed on thesubstrate 101 and may cover the bottom conductive layer 103. Theinsulation layer 105 may include, for example, oxides, nitrides,oxynitrides, silicates (e.g., metal silicates), aluminates, titanates,nitrides, high-k dielectric materials, or a combination thereof. Theinsulation layer 105 may be formed by suitable deposition processes, forexample, chemical vapor deposition, plasma-enhanced chemical vapordeposition, atomic layer deposition, evaporation, chemical solutiondeposition, or other suitable deposition processes. The thickness of theinsulation layer 105 may vary depending on the deposition process aswell as the composition and number of materials used. For example, thethickness of the insulation layer 105 may be between about 10 angstromsand about 50 angstroms. In some embodiments, the insulation layer 105may include multiple layers. For example, the insulation layer 105 maybe an oxide-nitride-oxide (ONO) structure. For another example, theinsulation layer 105 may include a bottom layer formed of silicon oxideand a top layer formed of high-k dielectric materials.

Examples of high-k dielectric materials (with a dielectric constantgreater than 7.0) include, but are not limited to, metal oxides such ashafnium oxide, hafnium silicon oxide, hafnium silicon oxynitride,lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconiumsilicon oxide, zirconium silicon oxynitride, tantalum oxide, titaniumoxide, barium strontium titanium oxide, barium titanium oxide, strontiumtitanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalumoxide, and lead zinc niobate. The high-k dielectric materials mayfurther include dopants such as, for example, lanthanum and aluminum.

In some embodiments, an interfacial layer (not shown) may be formedbetween the substrate 101 and the insulation layer 105. The interfaciallayer may include be formed of, for example, silicon oxide, siliconnitride, silicon oxynitride, other semiconductor oxides, or acombination thereof. The interfacial layer may be formed to any suitablethickness using any suitable process including thermal growth, atomiclayer deposition, chemical vapor deposition, high-density plasmachemical vapor deposition, spin-on deposition, or other suitabledeposition processes. For example, the thickness of the interfaciallayer may be between about 7 angstroms and 12 angstroms or between about8 angstroms and 10 angstroms. The interfacial layer may facilitate theformation of the insulation layer 105 during fabrication of thesemiconductor device 1A.

With reference to FIG. 1 and FIGS. 4 to 7, at step S15, a firstconductive layer 201, a second conductive layer 301, and a thirdconductive layer 401 may be formed on the insulation layer 105.

With reference to FIG. 4, intermediate conductive layers 601, 603, 605may be formed on the insulation layer 105 and may be directly above thebottom conductive layer 103. The intermediate conductive layers 601,603, 605 may be separated from each other. The intermediate conductivelayers 601, 603, 605 may be formed of, for example, undopedpolycrystalline silicon, undoped polycrystalline germanium, undopedpolycrystalline silicon germanium, or a combination thereof. It shouldbe noted that the number of the intermediate conductive layers is justfor exemplary purpose. The number of the intermediate conductive layersmay be greater than or less than three. For example, the number of theintermediate conductive layers can be two. For another example, thenumber of the intermediate conductive layers can be four.

With reference to FIG. 5, a mask layer 607 may be formed on theinsulation layer 105. The mask layer 607 may cover the intermediateconductive layers 603, 605 and expose the intermediate conductive layer601 (as shown in FIG. 4). In some embodiments, the mask layer 607 may bea photoresist layer. A first implantation process IMP1 may besubsequently performed to dope dopants into the intermediate conductivelayer 601 and turn the intermediate conductive layer 601 into the firstconductive layer 201. The dopants may be p-type dopants such as boron,aluminum, gallium, and indium or n-type dopants such as antimony,arsenic, and phosphorous. The dopant concentration of the firstimplantation process IMP1 may be between about 1E19 atoms/cm{circumflexover ( )}2 and about 1E21 atoms/cm{circumflex over ( )}2. The firstconductive layer 201 may have a first dopant concentration. After thefirst implantation process IMP1, the mask layer 607 may be removed.

With reference to FIG. 6, a mask layer 609 may be formed on theinsulation layer 105. The mask layer 609 may cover the intermediateconductive layer 605 and the first conductive layer 201 and expose theintermediate conductive layer 603 (as shown in FIG. 4). In someembodiments, the mask layer 609 may be a photoresist layer. A secondimplantation process IMP2 may be subsequently performed to dope dopantsinto the intermediate conductive layer 605 and turn the intermediateconductive layer 605 into the second conductive layer 301. The dopantsmay be p-type dopants such as boron, aluminum, gallium, and indium orn-type dopants such as antimony, arsenic, and phosphorous. The dopantconcentration of the second implantation process IMP2 may be betweenabout 1E19 atoms/cm{circumflex over ( )}2 and about 1E21atoms/cm{circumflex over ( )}2. In some embodiments, the dopant typeused in the first implantation process IMP1 and the dopant type used inthe second implantation process IMP2 are the same. In some embodiments,the dopants used in the first implantation process IMP1 and the dopantsused in the second implantation process IMP2 are the same. The secondconductive layer 301 may have a second dopant concentration. After thesecond implantation process IMP2, the mask layer 609 may be removed.

With reference to FIG. 7, a mask layer 611 may be formed on theinsulation layer 105. The mask layer 611 may cover the first conductivelayer 201 and the second conductive layer 301 and expose theintermediate conductive layer 605. In some embodiments, the mask layer611 may be a photoresist layer. A third implantation process IMP3 may besubsequently performed to dope dopants into the intermediate conductivelayer 605 and turn the intermediate conductive layer 605 into the thirdconductive layer 401. The dopants may be p-type dopants such as boron,aluminum, gallium, and indium or n-type dopants such as antimony,arsenic, and phosphorous. The dopant concentration of the thirdimplantation process IMP3 may be between about 1E19 atoms/cm{circumflexover ( )}2 and about 1E21 atoms/cm{circumflex over ( )}2. In someembodiments, the dopant type used in the third implantation process IMP3and the dopant type used in the second implantation process IMP2 are thesame. In some embodiments, the dopants used in the third implantationprocess IMP3 and the dopants used in the second implantation processIMP2 are the same. The third conductive layer 401 may have a thirddopant concentration. After the third implantation process IMP3, themask layer 611 may be removed.

In some embodiments, the first dopant concentration of the firstconductive layer 201, the second dopant concentration of the secondconductive layer 301, the third dopant concentration of the thirdconductive layer 401 may be different. For example, the third dopantconcentration of the third conductive layer 401 may be greater than thesecond dopant concentration of the second conductive layer 301 and thefirst dopant concentration of the first conductive layer 201. The seconddopant concentration of the second conductive layer 301 may be greaterthan the first dopant concentration of the first conductive layer 201.It should be note that the dopant concentration order of the firstconductive layer 201, the second conductive layer 301, and the thirdconductive layer 401 may be illustrative only. The dopant concentrationorder can be vary depending on circuit design.

In some embodiments, an anneal process may be performed to activate thefirst conductive layer 201, the second conductive layer 301, and thethird conductive layer 401. The temperature of the anneal process may bebetween about 800° C. and about 1250° C. The anneal process may have aprocess duration between about 1 millisecond and about 500 milliseconds.The anneal process may be, for example, a rapid thermal anneal, a laserspike anneal, or a flash lamp anneal.

With reference to FIGS. 1 and 8, at Step S17, a top conductive layer 111may be formed to electrically couple to the first conductive layer 201,the second conductive layer 301, and the third conductive layer 401 anda conductive plug 107 may be formed to electrically couple to the bottomconductive layer 103.

With reference to FIG. 8, an inter-dielectric layer 115 may be formed onthe insulation layer 105 to cover the first conductive layer 201, thesecond conductive layer 301, and the third conductive layer 401. Theinter-dielectric layer 115 may be formed of, for example, siliconnitride, silicon oxide, silicon oxynitride, flowable oxide, tonensilazen, undoped silica glass, borosilica glass, phosphosilica glass,borophosphosilica glass, plasma-enhanced tetraethyl orthosilicate,fluoride silicate glass, carbon-doped silicon oxide, or a combinationthereof. The inter-dielectric layer 115 may be formed by, for example,chemical vapor deposition, plasma-enhanced chemical vapor deposition, orthe like. A planarization process, such as chemical mechanicalpolishing, may be performed to provide a substantially flat surface forsubsequent processing steps.

With reference to FIG. 8, conductive vias 109 may be formed in theinter-dielectric layer 115. The conductive vias 109 may be respectivelyformed on the first conductive layer 201, the second conductive layer301, and the third conductive layer 401. In some embodiments, thesidewalls of each of the conductive vias 109 may have a slantedcross-sectional profile. In some embodiments, the conductive vias 109may be formed by a damascene method. Generally, in the damascene method,one or more dielectric materials, such as the low-k dielectric materials(i.e., having a dielectric constant <4.0), are deposited and patternetched to form the vertical interconnects, also known as vias, andhorizontal interconnects, also known as lines. Conductive materials,such as copper containing materials, and other materials, such asbarrier layer materials used to prevent diffusion of copper containingmaterials into the surrounding low-k dielectric, are then inlaid intothe etched pattern. Any excess copper containing materials and excessbarrier layer material-external to the etched pattern, such as on thefield of the substrate, is then removed.

With reference to FIG. 8, an inter-dielectric layer 117 may be formed onthe inter-dielectric layer 115. In some embodiments, theinter-dielectric layer 117 may be formed of a same material as theinter-dielectric layer 115 but is not limited thereto.

With reference to FIG. 8, the top conductive layer 111 may be formed inthe inter-dielectric layer 117. The top conductive layer 111 may beformed on the conductive vias 109. The top conductive layer 111 may beelectrically coupled to the first conductive layer 201, the secondconductive layer 301, and the third conductive layer 401 through theconductive vias 109. The top conductive layer 111 may be electricallycoupled to an external voltage during a programming procedure. The topconductive layer 111 may be formed of, for example, tungsten, cobalt,zirconium, tantalum, titanium, aluminum, ruthenium, copper, metalcarbides (e.g., tantalum carbide, titanium carbide, tantalum magnesiumcarbide), metal nitrides (e.g., titanium nitride), transition metalaluminides, or a combination thereof. The top conductive layer 111 maybe formed by, for example, a damascene process.

In some embodiments, the first conductive layer 201, the secondconductive layer 301, and the third conductive layer 401 may beelectrically coupled to different conductive layers and may beelectrically coupled different external voltages during programmingprocedures.

With reference to FIG. 8, an inter-dielectric layer 121 may be formed onthe inter-dielectric layer 117. In some embodiments, theinter-dielectric layer 121 may be formed of a same material as theinter-dielectric layer 117. In some embodiments, the inter-dielectriclayer 121 may be an etch stop layer and may be formed of, for example,silicon nitride, silicon carbonitride, or the like. The inter-dielectriclayer 121 may be formed by, for example, atomic layer deposition,chemical vapor deposition, or the like.

With reference to FIG. 8, the conductive plug 107 may be formed alongthe inter-dielectric layer 121, the inter-dielectric layer 117, theinter-dielectric layer 115, and the insulation layer 105, and on thebottom conductive layer 103. The conductive plug 107 may be electricallyconnected to the bottom conductive layer 103. The conductive plug 107may be electrically coupled to a biased voltage or ground potential. Insome embodiments, the conductive plug 107 may have a slantedcross-sectional profile. The conductive plug 107 may be formed of, forexample, tungsten, cobalt, zirconium, tantalum, titanium, aluminum,ruthenium, copper, metal carbides (e.g., tantalum carbide, titaniumcarbide, tantalum magnesium carbide), metal nitrides (e.g., titaniumnitride), transition metal aluminides, or a combination thereof.

With reference to FIG. 8, the first conductive layer 201, the secondconductive layer 301, the third conductive layer 401, the insulationlayer 105, and the bottom conductive layer 103 together configure aprogrammable unit. The configuration of the first conductive layer201—the insulation layer 105—the bottom conductive layer 103, theconfiguration of the second conductive layer 301—the insulation layer105—the bottom conductive layer 103, and the configuration of the thirdconductive layer 401—the insulation layer 105—the bottom conductivelayer 103 may be referred to as three capacitor-like structures (i.e.,conductor-insulator-conductor structure).

For a capacitor-like structure having n-type conductors, mobile carriers(electrons) may accumulate at the surface of the insulator when avoltage greater (i.e., larger in value) than the flat band voltage ofthe capacitor-like structure is applied. For a capacitor-like structurehaving p-type conductors, mobile carriers (holes) may accumulate at thesurface of the insulator when a voltage deeper (i.e., smaller in value)than the flat band voltage of the capacitor-like structure is applied.When mobile carriers of a capacitor-like structure are accumulated atthe surface of the insulator, the capacitor-like structure operates inaccumulation mode. As the flat hand voltage of a capacitor-likestructure is associated with the work function of the capacitor-likestructure and the work function of a capacitor-like structure isassociated with the dopant concentration of the capacitor-likestructure, the flat band voltage of a capacitor-like structure isassociated with the dopant concentration of the capacitor-likestructure.

In one embodiment of the present disclosure, the bottom conductive layer103 is formed of silicon doped with n-type dopant. The first conductivelayer 201, the second conductive layer 301, and the third conductivelayer 401 are formed of polycrystalline silicon doped with n-typedopant. The third dopant concentration of the third conductive layer 401may be greater than the second dopant concentration of the secondconductive layer 301. The second dopant concentration of the secondconductive layer 301 may be greater than the first dopant concentrationof the first conductive layer 201. In such embodiment, the first flatband voltage of the first conductive layer 201 is greater than thesecond flat band voltage of the second conductive layer 301 and thesecond flat band voltage of the second conductive layer 301 is greaterthan the third flat band voltage of the third conductive layer 401. Forexample, the first flat voltage of the first conductive layer 201 is 4.0volts, the second voltage of the second conductive layer 301 is 3.0volts, and the third voltage of the third conductive layer 401 is 2.0volts. As the flat band voltage of the first conductive layer 201, thesecond conductive layer 301, and the third conductive layer 401 aredifferent, different voltages may be used to respectively operate thefirst conductive layer 201, the second conductive layer 301, and thethird conductive layer 401 in the accumulation mode.

In a programming procedure of current embodiment, a programming voltagemay be applied to the semiconductor device 1A through the top conductivelayer 111 and the conductive plug 107 may be electrically coupled toground potential. The insulation layer 105 may be stressed under theprogramming voltage. As a result, the insulation layer 105 may beruptured to form a contiguous path(s) connecting the bottom conductivelayer 103 and the first conductive layer 201, the second conductivelayer 301, or the third conductive layer 401. By adjusting the value ofthe programming voltage, the number of the contiguous path(s) formed canbe controlled. That is, the resistance of the programmable unit afterthe programming procedure can be fine-tuned by controlling theprogramming voltage.

For example, when the programming voltage is 2.5 volts, only thecontiguous path of the third conductive layer 401 and the bottomconductive layer 103 is formed. For another example, when theprogramming voltage is 3.5 volts, the contiguous path of the thirdconductive layer 401 and the bottom conductive layer 103 and thecontiguous path of the second conductive layer 301 and the bottomconductive layer 103 are formed. For yet another example, when theprogramming voltage is 4.5 volts, all contiguous paths are formed.

In another embodiment of the present disclosure, the bottom conductivelayer 103 is formed of silicon doped with p-type dopant. The firstconductive layer 201, the second conductive layer 301, and the thirdconductive layer 401 are formed of polycrystalline silicon doped withp-type dopant. The third dopant concentration of the third conductivelayer 401 may be greater than the second dopant concentration of thesecond conductive layer 301. The second dopant concentration of thesecond conductive layer 301 may be greater than the first dopantconcentration of the first conductive layer 201. In such embodiment, thefirst flat band voltage of the first conductive layer 201 is shallower(i.e., closer to ground potential) than the second flat band voltage ofthe second conductive layer 301 and the second flat band voltage of thesecond conductive layer 301 is shallower than the third flat bandvoltage of the third conductive layer 401. For example, the first flatvoltage of the first conductive layer 201 is −2.0 volts, the secondvoltage of the second conductive layer 301 is −3.0 volts, and the thirdvoltage of the third conductive layer 401 is −4.0 volts. As the flatband voltage of the first conductive layer 201, the second conductivelayer 301, and the third conductive layer 401 are different, differentvoltages may be used to respectively operate the first conductive layer201, the second conductive layer 301, and the third conductive layer 401in the accumulation mode.

Exemplary programming procedures may be as follow. When the programmingvoltage is −2.5 volts, only the contiguous path of the third conductivelayer 401 and the bottom conductive layer 103 is formed. For anotherexample, when the programming voltage is −3.5 volts, the contiguous pathof the third conductive layer 401 and the bottom conductive layer 103and the contiguous path of the second conductive layer 301 and thebottom conductive layer 103 are formed. For yet another example, whenthe programming voltage is −4.5 volts, all contiguous paths are formed.

FIGS. 9 to 13 illustrate, in schematic cross-sectional. view diagrams,semiconductor devices 1B, 1C, 1D, 1E, and 1F in accordance with someembodiments of the present disclosure.

With reference to FIG. 9, the semiconductor device 1B may have astructure similar to that illustrated in FIG. 8. The same or similarelements in FIG. 9 as in FIG. 8 have been marked with similar referencenumbers and duplicative descriptions have been omitted. Thesemiconductor device 1B may include a well region 125. The well region125 may be disposed in the substrate 101 and may surround the bottomconductive layer 103. The well region 125 may have an electrical typeopposite to the bottom conductive layer 103. The well region 125 mayprovide additional electrical isolation to the bottom conductive layer103.

With reference to FIG. 10, the semiconductor device 1C may have astructure similar to that illustrated in FIG. 8. The same or similarelements in FIG. 10 as in FIG. 8 have been marked with similar referencenumbers and duplicative descriptions have been omitted. Thesemiconductor device 1C may include assistance layers 123. Theassistance layers 123 may be respectively correspondingly disposedcovering the first conductive layer 201, the second conductive layer301, and the third conductive layer 401. The assistance layers 123 maybe formed of, for example, titanium silicide, nickel silicide, nickelplatinum silicide, tantalum silicide, or cobalt silicide. The thicknessof the assistance layers 123 may be between about 2 nm and about 20 nm.The assistance layers 123 may reduce the contact resistance between theconductive vias 109 and the first conductive layer 201, the secondconductive layer 301, and the third conductive layer 401, respectively.In some embodiments, the assistance layers 123 may be disposed on thetop surface of the first conductive layer 201, the top surface of thesecond conductive layer 301, and the top surface of the third conductivelayer 401, respectively.

With reference to FIG. 11, the semiconductor device 1D may have astructure similar to that illustrated in FIG. 8. The same or similarelements in FIG. 11 as in FIG. 8 have been marked with similar referencenumbers and duplicative descriptions have been omitted.

The semiconductor device 1D may include spacers 113. The spacers 113 maybe disposed on the sidewalls of the first conductive layer 201, thesidewalls of the second conductive layer 301, and the sidewalls of thesecond conductive layer 301, respectively. In some embodiments, thespacers 113 may be formed of, for example, silicon oxide, siliconnitride, silicon oxynitride, silicon nitride oxide, the like, or acombination thereof. The spacers 113 may provide additional electricalisolation to the first conductive layer 201, the second conductive layer301, and the third conductive layer 401. In some embodiments, thespacers 113 may be formed of low-k dielectric materials or porousdielectric material. The spacers 113 formed of low-k dielectricmaterials or porous dielectric material may reduce parasitic capacitancebetween the first conductive layer 201 and the second conductive layer301 and between the second conductive layer 301 and the third conductivelayer 401.

With reference to FIG. 12, the semiconductor device 1E may have astructure similar to that illustrated in FIG. 8. The same or similarelements in FIG. 12 as in FIG. 8 have been marked with similar referencenumbers and duplicative descriptions have been omitted. The bottomconductive layer 103 of the semiconductor device 1E may be formed of,for example, a metallic material. The metallic material may be, forexample, tungsten, cobalt, zirconium tantalum, titanium, aluminum,ruthenium, copper, or a combination thereof.

With reference to FIG. 13, the semiconductor device 1F may have astructure similar to that illustrated in FIG. 8. The same or similarelements in FIG. 13 as in FIG. 8 have been marked with similar referencenumbers and duplicative descriptions have been omitted.

The first conductive layer 201, the second conductive layer 301, and thethird conductive layer 401 may be formed of, for example, metallicmaterials such as tungsten, cobalt, zirconium, tantalum, titanium,aluminum, ruthenium, copper, platinum, or a combination thereof. Thefirst conductive layer 201, the second conductive layer 301, and thethird conductive layer 401 may be formed of different materials so as tohave different work functions. For example, the first conductive layer201 may be formed of aluminum and having a work function at +4.1 volts.The second conductive layer 301 may be formed of copper and having awork function at +4.7 volts. The third conductive layer 401 may beformed of platinum and having a work function at +6.4 volts. As thedifferent work functions of the first conductive layer 201, the secondconductive layer 301, and the third conductive layer 401, differentprogramming voltages may be needed to program the first conductive layer201, the second conductive layer 301, or the third conductive layer 401.

FIG. 14 illustrates, in a flowchart diagram form, a method 20 forfabricating a semiconductor device 1G in accordance with anotherembodiment of the present disclosure. FIGS. 15 to 28 illustrate, inschematic cross-sectional view diagrams, a flow for fabricating thesemiconductor device 1G in accordance with another embodiment of thepresent disclosure.

With reference to FIGS. 14 and 15, at step S21, a substrate 101 may beprovided, a bottom conductive layer 103 may be formed in the substrate101, pseudo-conductive layers 613 may be formed on the bottom conductivelayer 103, and hard mask layers 615 may be finned on thepseudo-conductive layers 613.

With reference to FIG. 15, the substrate 101 and the bottom conductivelayer 103 may be formed with a procedure similar to that illustrated inFIG. 2. The pseudo-conductive layers 613 may be separated from eachother. The pseudo-conductive layers 613 may be formed of, for example,amorphous silicon or polycrystalline silicon. The hard mask layers 615may be formed of, for example, silicon nitride, silicon oxynitride,silicon nitride oxide, or the like.

With reference to FIGS. 14 and 16, at step S23, spacers 113 may beformed on sidewalls of the pseudo-conductive layers 613 and sidewalls ofthe hard mask layers 615 and an inter-dielectric layer 115 may be formedto cover the spacers 113 and the hard mask layers 615.

With reference to FIG. 16, a layer of insulating material may bedeposited to cover the pseudo-conductive layers 613 and the hard masklayers 615. An etch process, such as an anisotropic dry etch process,may be subsequently performed to remove portions of the layer ofinsulating material and concurrently form the spacers 113. Theinter-dielectric layer 115 may be formed by a deposition process such aschemical vapor deposition. A planarization process, such as chemicalmechanical polishing, may be performed onto the inter-dielectric layer115 to provide a substantially flat surface for subsequent processingsteps.

With reference to FIGS. 14, 17, and 18, at Step S25, removing theintermediate conductive layer 603 to form first trench 701, secondtrench 703, and third trench 705 in the inter-dielectric layer 115 toexpose portions of the bottom conductive layer 103.

With reference to FIG. 17, a planarization process, such as chemicalmechanical polishing, may be performed to remove the hard mask layers615 and portions of the spacers 113 and to expose the pseudo-conductivelayers 613.

With reference to FIG. 18, an etch process, such as an anisotropic thyetch process, may be performed to remove the pseudo-conductive layers613 and concurrently form the first trench 701, the second trench 703,and the third trench 705. Portions of the top surface of bottomconductive layer 103 may be exposed through the first trench 701, thesecond trench 703, and the third trench 705.

With reference to FIG. 14 and FIGS. 19 to 27, at Step S27, a

first gate structure 200 may be formed in the first trench 701, a secondgate structure 300 may be formed in the second trench 703, and a thirdgate structure 400 may be formed in the third trench 705.

With reference to FIG. 19, a layer of dielectric material 617 may beconformally formed over the intermediate semiconductor deviceillustrated in FIG. 18. The dielectric material 617 may be, for example,silicon oxide, silicon nitride, hafnium oxide, hafnium silicon oxide,hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide,zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride,tantalum oxide, titanium oxide, barium strontium titanium oxide, bariumtitanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide,lead scandium tantalum oxide, or lead zinc niobate.

With reference to FIG. 20, a layer of first work function material 619may be conformally formed on the layer of dielectric material 617. Thefirst work function material 619 may be, for example, ruthenium,palladium, platinum, cobalt, nickel, and conductive metal oxides,titanium nitride, hafnium, zirconium, titanium, tantalum, aluminum,metal carbides (e.g., hafnium carbide, zirconium carbide, titaniumcarbide, and aluminum carbide), aluminides, or a combination thereof. Amask layer 627 may be formed on the layer of first work functionmaterial 619 to cover the third trench 705 and expose the second trench703 and the first trench 701.

With reference to FIG. 21, an etch process may be performed toselectively remove the exposed portion of the layer of first workfunction material 619. The etch rate ratio of the first work functionmaterial 619 to the dielectric material 617 may be between about 15:1and about 2:1 or between about 10:1 and about 3:1 during the etchprocess. After the etch process, the mask layer 627 may be removed.

With reference to FIG. 22, a layer of second work function material 621may be conformally formed on the layer of first work function material619 and the layer of dielectric material 617. The second work functionmaterial 621 may be different from the first work function material 619.The second work function material 621 may be, for example, ruthenium,palladium, platinum, cobalt, nickel, and conductive metal oxides,titanium nitride, hafnium, zirconium, titanium, tantalum, aluminum,metal carbides, aluminides, or a combination thereof. A mask layer 629may be formed on the layer of second work function material 621 to coverthe second trench 703 and expose the third trench 705 and the firsttrench 701.

With reference to FIG. 23, an etch process may be performed toselectively remove the exposed portion of the layer of second workfunction material 621. The etch rate ratio of the second work functionmaterial 621 to the dielectric material 617 may be between about 15:1and about 2:1 or between about 10:1 and about 3:1 during the etchprocess. The etch rate ratio of the second work function material 621 tothe first work function material 619 may be between about 15:1 and about2:1 or between about 10:1 and about 3:1 during the etch process. Afterthe etch process, the mask layer 629 may be removed.

With reference to FIG. 24, a layer of third work function material 623may be conformally formed on the layer of first work function material619, the layer of second work function material 621, and the layer ofdielectric material 617. The third work function material 623 may bedifferent from the first work function material 619 and the second workfunction 621. The third work function material 623 may be, for example,ruthenium, palladium, platinum, cobalt, nickel, and conductive metaloxides, titanium nitride, hafnium, zirconium, titanium, tantalum,aluminum, metal carbides, aluminides, or a combination thereof. A masklayer 631 may be formed on the layer of third work function material 623to cover the first trench 701 and expose the second trench 703 and thethird trench 705.

With reference to FIG. 25, an etch process may be performed toselectively remove the exposed portion of the layer of third workfunction material 623. The etch rate ratio of the third work functionmaterial 623 to the dielectric material 617 may be between about 15:1and about 2:1 or between about 10:1 and about 3:1 during the etchprocess. The etch rate ratio of the third work function material 623 tothe first work function material 619 may be between about 15:1 and about2:1 or between about 10:1 and about 3:1 during the etch process. Theetch rate ratio of the third work function material 623 to the secondwork function material 621 may be between about 15:1 and about 2:1 orbetween about 10:1 and about 3:1 during the etch process. After the etchprocess, the mask layer 631 may be removed.

With reference to FIG. 26, a layer of filler material 625 may be formedto fill the first trench 701, the second trench 703, and the secondtrench 703. The filler material 625 may be, for example, tungsten,aluminum, cobalt, ruthenium, gold, silver, titanium, platinum, the like,or a combination thereof. The layer of filler material 625 may be formedby, for example, chemical vapor deposition, physical vapor deposition,plating, thermal or e-beam evaporation, the like, or a combinationthereof.

With reference to FIG. 27, a planarization process, such as chemicalmechanical polishing, may be performed until the top surface of theinter-dielectric layer 115 is exposed. After the planarization process,the layer of dielectric material 617 may be turned into a first gatedielectric layer 203 in the first trench 701, a second gate dielectriclayer 303 in the second trench 703, and a third gate dielectric layer403 in the third trench 705. The layer of first work function material619 may be turned into a third work function layer 405 in the thirdtrench 705. The layer of second work function material 621 may be turnedinto a second work function layer 305 in the second trench 703. Thelayer of third work function material 623 may be turned into a firstwork function layer 205 in the first trench 701. The layer of fillermaterial 625 may be turned into a first filler layer 207 in the firsttrench 701, a second filler layer 307 in the second trench 703, and athird filler layer 407 in the third trench 705. The thicknesses of thefirst gate dielectric layer 203, the second gate dielectric layer 303,and the third gate dielectric layer 403 may be the same.

With reference to FIG. 27, the first gate dielectric layer 203, thefirst work function layer 205, the second gate dielectric layer 303, thesecond work function layer 305, the third gate dielectric layer 403, andthe third work function layer 405 may have U-shaped cross-sectionalprofiles.

With reference to FIG. 27, the first gate dielectric layer 203, thefirst work function layer 205, and the first filler layer 207 togetherconfigure the first gate structure 200. The second gate dielectric layer303, the second work function layer 305, and the second filler layer 307together configure the second gate structure 300. The third gatedielectric layer 403, the third work function layer 405, and the thirdfiller layer 407 together configure the third gate structure 400. Insome embodiments, the fabrication of the first gate structure 200, thesecond gate structure 300, and the third gate structure 400 may beeasily integrated with gates of logic elements

With reference to FIGS. 14 and 28, at Step S29, a conductive plug 107may be formed to electrically couple to the bottom conductive layer 103and a top conductive layer 111 may be firmed to electrically couple tothe first gate structure 200, the second gate structure 300, and thethird gate structure 400.

With reference to FIG. 28, inter-dielectric layers 117 may be formed onthe inter-dielectric layer 115 and inter-dielectric layers 119 may beformed on the inter-dielectric layer 117. The conductive vias 109 andthe top conductive layer 111 may be formed in the inter-dielectric layer117 and the inter-dielectric layer 119 by, for example, a damasceneprocess. The top conductive layer 111 may be electrically coupled to thefirst gate structure 200, the second gate structure 300, and the thirdgate structure 400 through the conductive vias 109. The inter-dielectriclayer 121 may be formed on the inter-dielectric layer 119. Theconductive plug 107 may be formed along the inter-dielectric layers 115,117, 119, 121 and on the bottom conductive layer 103.

As the first work function layer 205, the second work function layer305, and the third work function layer 405 are formed of different workfunction material, the first gate structure 200, the second gatestructure 300, and the third gate structure 400 have different workfunctions. Accordingly, different programming voltages may be needed toprogram the first gate structure 200, the second gate structure 300, orthe third gate structure 400.

FIGS. 29 and 30 illustrate, in schematic cross-sectional view diagrams,semiconductor devices 1H and 1I in accordance with some embodiments ofthe present disclosure.

With reference to FIG. 29, the semiconductor device 1H may have astructure similar to that illustrated in FIG. 28. The same or similarelements in FIG. 29 as in FIG. 28 have been marked with similarreference numbers and duplicative descriptions have been omitted. Thefirst gate structure 200 may further include first wetting layer 209 andfirst barrier layer 211. The second gate structure 300 may furtherinclude second wetting layer 309 and second barrier layer 311. The thirdgate structure 400 may further include third wetting layer 409 and thirdbarrier layer 411.

The first wetting layer 209 may be disposed between the first workfunction layer 205 and the first filler layer 207. The second wettinglayer 309 may be disposed between the second work function layer 305 andthe second filler layer 307. The third wetting layer 409 may be disposedbetween the third work function layer 405 and the third filler layer407. The first wetting layer 209, the second wetting layer 309, and thethird wetting layer 409 may be formed of, for example, titanium,tantalum, nickel, or cobalt. The first wetting layer 209, the secondwetting layer 309, and the third wetting layer 409 may promote bondingbetween the layers and may promote uniform deposition of the subsequentlayers.

The first barrier layer 211 may be disposed between the first wettinglayer 209 and the first filler layer 207. The second barrier layer 311may be disposed in the second wetting layer 309 and the second fillerlayer 307. The third barrier layer 411 may be disposed between the thirdwetting layer 409 and the third filler layer 407. The first barrierlayer 211, the second barrier layer 311, and the third barrier layer 411may be formed of, for example, titanium nitride, tantalum nitride, or acombination thereof. The first barrier layer 211, the second barrierlayer 311, and the third barrier layer 411 may prevent subsequentdeposition processes from degrading other layers of the semiconductordevice 1H.

With reference to FIG. 30, the semiconductor device 1I may have astructure similar to that illustrated in FIG. 28. The same or similarelements in FIG. 30 as in FIG. 28 have been marked with similarreference numbers and duplicative descriptions have been omitted.

With reference to FIG. 30, the substrate 101, the bottom conductivelayer 103, and the insulation layer 105 may be formed with a proceduresimilar to that illustrated in FIGS. 2 and 3. The inter-dielectric layer115 and the spacers 113 may be formed with a procedure similar to thatillustrated in FIGS. 15 to 18. The first work function layer 205, thefirst filler layer 207, the second work function layer 305, the secondfiller layer 307, the third work function layer 405, the third fillerlayer 407, the conductive plug 107, the conductive vias 109, the topconductive layer 111, and the inter-dielectric layers 117, 119, 121 maybe formed with a procedure similar to that illustrated in FIGS. 20 to28.

With reference to FIG. 30, the first work function layer 205 and thefirst filler layer 207 together configure the first gate structure 200.The second work function layer 305 and the second filler layer 307together configure the second gate structure 300. The third workfunction layer 405 and the third filler layer 407 together configure thethird gate structure 400. As the first work function layer 205, thesecond work function layer 305, and the third work function layer 405are formed of different work function material, the first gate structure200, the second gate structure 300, and the third gate structure 400have different work functions. Accordingly, different programmingvoltages may be needed to program the first gate structure 200, thesecond gate structure 300, or the third gate structure 400.

Due to the design of the semiconductor device 1A of the presentdisclosure, the resistance of the programmable unit after a programmingprocedure can be fine-tuned by controlling the programming voltageapplied. In addition, various of programming voltages (e.g., −4.5 voltsto +4.5 volts) can be selected and applied to program the programmableunit. In other words, the programmable unit can be operated with variousvoltages in an integrated circuit according to design demand.Furthermore, the programmable unit can be programmed by a relativelysmaller (or shallower) voltage (e.g., −2.5 volts or +2.5 volts). Thatis, the surface area fir a charge-pump circuit, which is used togenerate the programming voltage, can be reduced. As a result, moresurface area can be provided for other functional elements such as logicfunction elements.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutions,and alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. For example,many of the processes discussed above can be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, and steps.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a bottom conductive layer positioned in the substrate; an insulationlayer positioned on the substrate; a first conductive layer positionedon the insulation layer and above the bottom conductive layer; a secondconductive layer positioned on the insulation layer and above the bottomconductive layer and spaced apart from the first conductive layer; aconductive plug electrically coupled to the bottom conductive layer; anda top conductive layer electrically coupled to the first conductivelayer and the second conductive layer; wherein the first conductivelayer has a first work function and the second conductive layer has asecond work function different from the first work function; wherein thebottom conductive layer, the insulation layer, the first conductivelayer, and the second conductive layer together configure a programmableunit.
 2. The semiconductor device of claim 1, wherein the firstconductive layer and the second conductive layer are formed of dopedpolycrystalline silicon, doped polycrystalline silicon germanium, or acombination thereof, and the first conductive layer and the secondconductive layer have a same electrical type.
 3. The semiconductordevice of claim 2, wherein the bottom conductive layer is formed ofdoped silicon, doped germanium, doped silicon germanium, or acombination thereof, and the bottom conductive layer has a sameelectrical type as the first conductive layer and the second conductivelayer.
 4. The semiconductor device of claim 3, further comprising a wellregion positioned in the substrate and surrounding the bottom conductivelayer, wherein the well region has an electrical type opposite to thatof the bottom conductive layer.
 5. The semiconductor device of claim 3,further comprising assistance layers covering the first conductive layerand the second conductive layer, wherein the assistance layers areformed of titanium silicide, nickel silicide, nickel platinum silicide,tantalum silicide, or cobalt silicide.
 6. The semiconductor device ofclaim 3, further comprising spacers positioned on sidewalls of the firstconductive layer and sidewalls of the second conductive layer.
 7. Thesemiconductor device of claim 2, wherein the bottom conductive layer isformed of tungsten, aluminum, titanium, copper, or a combinationthereof.
 8. The semiconductor device of claim 1, wherein the firstconductive layer and the second conductive layer are formed of differentmaterials, the first conductive layer is formed of tungsten, cobalt,zirconium, tantalum, titanium, aluminum, ruthenium, copper, platinum, ora combination thereof, and the second conductive layer is formed oftungsten, cobalt, zirconium, tantalum, titanium, aluminum, ruthenium,copper, platinum, or a combination thereof.
 9. A semiconductor device,comprising: a substrate; a bottom conductive layer positioned in thesubstrate; a first gate structure comprising a first gate dielectriclayer positioned on the bottom conductive layer, a first work functionlayer positioned on the first gate dielectric layer, and a first fillerlayer positioned on the first work function layer, wherein the firstwork function layer has a first work function; a second gate structurecomprising a second gate dielectric layer positioned on the bottomconductive layer and spaced apart from the first gate dielectric layer,a second work function layer positioned on the second gate dielectriclayer, and a second filler layer positioned on the second work functionlayer, wherein the second work function layer has a second work functiondifferent from the first work function; a conductive plug electricallycoupled to the bottom conductive layer; and a top conductive layerelectrically coupled to the first gate structure and the second gatestructure; wherein the bottom conductive layer, the insulation layer,the first gate structure, and the second gate structure togetherconfigure a programmable unit.
 10. The semiconductor device of claim 9,wherein the first gate dielectric layer and the second gate dielectriclayer have a same thickness.
 11. The semiconductor device of claim 10,further comprising spacers positioned on two sides of the first gatestructure and on two sides of the second gate structure.
 12. Thesemiconductor device of claim 11, further comprising a first wettinglayer positioned between the first work function layer and the firstfiller layer, wherein the first wetting layer is formed of titanium,tantalum, nickel, or cobalt.
 13. The semiconductor device of claim 12,further comprising a first barrier layer positioned between the firstwetting layer and the first filler layer, wherein the first barrierlayer is formed of titanium nitride, tantalum nitride, or a combinationthereof.
 14. The semiconductor device of claim 9, wherein the first gatedielectric layer and the second gate dielectric layer have U-shapedcross-sectional profiles.
 15. The semiconductor device of claim 9,wherein the bottom conductive layer is formed of doped silicon, dopedgermanium, doped silicon germanium, or a combination thereof.
 16. Thesemiconductor device of claim 9, wherein the bottom conductive layer isformed of tungsten, aluminum, titanium, copper, or a combinationthereof.
 17. A semiconductor device, comprising: a substrate; a bottomconductive layer positioned in the substrate; an insulation layerpositioned on the substrate; a first gate structure positioned on theinsulation layer and above the bottom conductive layer and comprising afirst work function layer and a first filler layer, wherein the firstwork function layer has a first work function; a second gate structurepositioned on the insulation layer and above the bottom conductivelayer, spaced apart from the first gate structure, and comprising asecond work function layer and a second filler layer, wherein the secondwork function layer has a second work function different from the firstwork function; a conductive plug electrically coupled to the bottomconductive layer; and a top conductive layer electrically coupled to thefirst gate structure and the second gate structure; wherein the bottomconductive layer, the insulation layer, the first gate structure, andthe second gate structure together configure a programmable unit.
 18. Amethod for fabricating a semiconductor device, comprising: providing asubstrate; forming a bottom conductive layer in the substrate; formingan insulation layer on the substrate; forming a first conductive layeron the insulation layer and above the bottom conductive layer, whereinthe first conductive layer has a first work function; forming a secondconductive layer on the insulation layer and above the bottom conductivelayer, spaced apart from the first conductive layer, and having a secondwork function different from the first work function; wherein the bottomconductive layer, the insulation layer, the first conductive layer, andthe second conductive layer together configure a programmable unit. 19.The method for fabricating the semiconductor device of claim 18, whereinthe steps of forming the first conductive layer and forming the secondconductive layer comprise: forming a first intermediate conductive layerand a second intermediate conductive layer on the insulation layer;forming a first mask layer to cover the second intermediate conductivelayer and expose the first intermediate conductive layer; performing afirst implantation process to turn the first intermediate conductivelayer into the first conductive layer; removing the first mask layer;forming a second mask layer to cover the first conductive layer andexpose the second intermediate conductive layer; performing a secondimplantation process to turn the second intermediate conductive layerinto the second conductive layer; and removing the second mask layer;wherein the first implantation process and the second implantationprocess are performed with different dopant concentrations.
 20. Themethod for fabricating the semiconductor device of claim 19, wherein thebottom conductive layer, the first conductive layer, and the secondconductive layer have a same electrical type.